New ultra high technology aircraft and launch vehicles are presently being designed and developed. An example of such an aircraft is the "Orient Express". It is anticipated that the Orient Express would be able to achieve speeds of approximately Mach 8 and be able to fly from New York to Tokyo in approximately 3 hours. The feasibility of such an aircraft, however, depends on the development of new, strong, lightweight structural materials. Weight saving from the utilization of such materials directly translate into increases in payload as well as in fuel economy. In addition, engine materials must be developed that can withstand extremely high operating temperatures up to, for example, 3,200.degree. F. Such high operating temperatures are preferred as they increase overall fuel efficiency.
Because of the unique requirements for the construction of such a sustained high speed aircraft, progress must be made in the development of new composites and advanced alloys from which turbine blades and other engine components may be constructed. At present, the most likely candidates for these advanced alloys may be found in the metal matrix composites with non-metallic elements or rare earth elements and intermetallic compounds of transition metals with aluminum such as titanium and tantalum aluminides.
Titanium alloys have historically been attractive for use in gas turbine engines because of their low density, high strength and good corrosion resistance. These properties have permitted the substitution of titanium alloys for heavier, less effective materials. In many cases, titanium alloys have allowed engineers to not only increase the strength of aircraft but also reduce the overall weight as well as the number of parts required. As such, titanium alloys are responsible for significant advances made in the construction of aircraft in recent years. Additional advances are, however, still required for supersonic aircraft designs such as the Orient Express.
More recently, powder metallurgy and rapid solidification techniques have been utilized to develop improved alloys. These techniques have originally shown promise as they facilitate greater amounts of alloying additions as well as improved homogeneity in the resulting product. They have also suggested that higher temperature applications may be attainable.
The initial stages of processing require the desired alloy to be melted and cast into a proper ingot in a protective atmosphere. This ingot is then atomized in an inert atmosphere with a high cooling rate by irradiating electron or laser beams over the surface of the rapidly rotating ingot. The alloy powders thus produced are then vacuum canned and hot deformed to achieve approximately 100% density.
Unfortunately, a number of processing problems have largely prevented these techniques from being effectively utilized on any type of successful commercial scale. More specifically, alloys prepared in the manner described above experience a number of difficulties including chemical segregation and contamination of the alloy from the crucible material during melting. More specifically, even though the cooling rate is maintained high during atomization to avoid chemical segregation within individual powder particles, the atomization process does not guarantee the chemical homogeneity in the entire powder product. This, of course, is true since the electron or laser beams melt only the skin of a rapidly rotating ingot. Thus, the inhomogeneity of an ingot inherent in this slow solidification is passed on to the resulting alloy powders.
Additional processing problems are associated with the powder metallurgy consolidation of the alloy powders to bulk products. Because of the reactivity of the alloy powders, the powders have to be vacuum canned and mechanically processed at elevated temperatures by, for example, pressing, forging, rolling, swaging, extrusion or hipping. All of these processing procedures require the removal of can material prior to further machining into products. Of course, heatings during this consolidation process require a long period of time to homogenize the temperature of powders loosely packed. Unfortunately, this serves to alter the microstructure of the alloy powders with a resulting degradation in the alloy properties of the product.
A need is therefore identified for an improved method for synthesizing high temperature alloys that retain their unique mechanical properties.